This invention relates to novel chromogenic compounds for use in recording sheet which develops color images by an electron donor-acceptor color-forming reaction between chromogenic compounds and acidic materials which react upon contact to produce a color.
More particularly, this invention relates to chromogenic compounds which can be classified as benz [a] fluorans, methods for the preparation thereof and the recording sheets containing the same as chromogenic materials.
Heretofore oil carbon papers have been widely used for duplicating documents and slips as office work materials. The carbon paper duplicating system is advantageous in that it is liable to stain clothes and hands and in that it is troublesome to insert the oil carbon paper between documents and slips.
In order to eliminate the inefficiencies and defects of the oil carbon paper, pressure-sensitive copying paper or noncarbon paper, which is economic and handy, has recently made its debut in place of the oil carbon paper complying with demand for speed-up of office works.
In the pressure-sensitive recording sheet utilized is an electron donor-acceptor color-forming reaction between colorless chromogenic materials such as crystal violet lactone and acidic electron acceptor such as acid clay, attapalgite, zeolite, kaolin, organic acid (e.g. salicylic acid), bisphenol-A, phenol-formalin resin, hydrolized maleic anhydride-styrene copolymer. Triphenylmethane compounds such as crystal violet lactone are usually used as chromogenic compounds for such purpose. Crystal violet lacone changes in color from colorless to vivid bluish violet when it is brought to contact with the electron-acceptor.
The triphenylmethane compounds used in the above mentioned pressure sensitive recording sheet have an inherent disadvantage that they are so unstable against sunlight that the colored images formed in contact with an electron-acceptor tend to disappear by the effect of sunlight or ultraviolet light in a short period of time. Further, triphenyl methane lactones are liable to be affected by water or moisture, that is, the droplets of water prevent the lactone ring from cleavage, so that the color images obtained by the opening of lactone ring tend to disappear by the effect of water or moisture.
Recently, attempts have been made to obtain improved pressure-sensitive copy sheets which provide black color images. For this purpose, several chromogenic compounds which produce various colors, for example, red, blue, yellow and etc. on contact with an electron-acceptor are mixed to use.
But this method is disadvantageous in that the process for making a coating color is so complicated and in that those various chromogenic compounds have different color deepness its color shade and different color developing speed, due to their different basic structures, therefore, the produced color images tend to change in shade, e.g., from black to reddish black, as time passes.
The primary object of the invention is to provide novel chromogenic compounds in which the above mentioned disadvantages can be avoided.
Another object of the invention is to provide useful methods for preparing such novel compounds.
A further object of the invention is to provide new and improved markforming record sheet utilizing chromogenic compounds which react with acidic materials upon contact to produce colors.
Other objects of the invention will be partly obvious and will partly become apparent from the following descriptions.